Dulling hard aluminum oxide coatings on aluminum-rich basis metals



United States Patent 3 140,245 DULLING HARD ALIiMINUM OXIDE COATINGS ONALUMINUM-RICH BASIS METALS Anthony Gallaccio, Havertown, and Reyburn W.Wick, Philadelphia, Pa., assignors to the United States of America asrepresented by the Secretary of the Army No Drawing. Filed Aug. 23,1961, Ser. No. 134,055 3 Claims. (Cl. 20435) (Granted under Title 35,US. Code (1952), sec. 266) The invention described herein may bemanufactured and used by or for the Goverment for governmental purposeswithout the payment to us of any royalty thereon.

This invention relates to aluminum and alloys thereof in which aluminumis the principal ingredient. It is particularly directed to theprovision of a process and an aqueous acid solution therefor whichproduce a dull, non-reflective, dye-receptive surface on hard anodizedaluminum oxide coatings on aluminum.

It is well known that metal articles having dark lustrous finishes aredesirable and suitable for numerous applications and uses, particularlyfor decorative purposes, to promote radiation, etc. Too, matte blackfinishes are important in providing non-reflecting surfaces inindustrial and military instruments of various types. However, on hardanodized coatings which have been electrolytically applied to aluminumand its alloys, a brownish, compact, high corrosion and erosionresistant coating has resulted. Since these coatings are dense andcompact, they are glossy and appreciably reflective, and hence aregenerally non-receptive to subsequent dyeing processes. In variedmilitary applications, the reflectivity of these hard anodized coatingsis undesirable.

It is recognized that the dulling of hard anodized aluminum oxidecoating surfaces might be accomplished by mechanical means, i.e., byabrasive blasting and the like, but a non-uniform reduction of coatingthickness or complete removal of the coating in areas frequentlyresults. Additionally, the dye receptivity of mechanically dulledsurfaces is not considered good.

It is therefore a broad object of the invention to provide means fordulling hard anodized aluminum oxide coatings on aluminum rich basismetals.

Another object of the invention is to provide a process and bath forrendering aluminum oxide coatings on aluminum-rich basis metals dull anddye-receptive.

Other and further objects of the invention will be apparent to thoseskilled in the art upon study of this disclosure.

We have discovered that black dyes, such, for example, as nigrosine andAluminum Black 3B, are extremely adherent to hard anodized aluminumoxide surfaces when dulled by the bath process of our invention.

One process, in accordance with our invention, referred to by Way ofexample, consists of cleaning the article and degreasing if necessary,and immersing in our treating solution for approximately 80 seconds, thesolution being maintained at about 95 C. and containing phosphoric acid,chromium tri-oxide and a suitable surface-active agent, such that the pHof the solution does not exceed about 2.0. Our process has beensuccessfully applied to a variety of commonly used alloys consistinglargely of aluminum as well as pure aluminum. A few of the well knownaluminum-rich alloys include at least one "ice of the metals selectedfrom manganese, magnesium, silicon, chromium and copper.

The solution used in practicing our improved dulling process arecharacterized by a content of acid, chromium tri-oxide and asurface-active agent which are within a rather Well-defined area, thefollowing being typical examples:

Formula No. 1

Phosphoric acid (H PO ml./l 30 Chromium tri-oxide (CrO g./l 15 Water, tomake 1 liter Balance Surface-active agent percent by weight 0.01

The surface-active agent may suitably be an alkylaryl polyether alcohol,such as Triton, or an alkylarylsulfonate such as Ultrawet.

A number of alternative bath formulas are given below illustrating a fewof the many variations in compositions Which our improved solutions maytake Within their operating range:

Formula N0 2 Phosphoric acid (H PO 85% ml./l 24 Chromium tri-oxide (CrOg./l 18 Water, to make 1 liter Balance Surface-active agent percent byweight 0.01

Formula No. 3

Phosphoric acid (H PO 85% ml./l 36 Chromium tri-oxide (CrO g./l 12Water, to make 1 liter Balance Surface-active agent percent by weight0.01

It is therefore possible to vary the phosphoric acid range between about24 to 36 ml./l. (equivalent to a molarity of about 0.38 to 0.57) and thechromium trioxide between about '12 to 18 g./l (equivalent to a molarityof about 0.12 to 0.18), the surface-active agent remaining about 0.01%by weight.

Of the examples given, Formula No. 1 produced excellent dulling of thehard anodized aluminum oxide coatings whereas Formulas No. 2 and No. 3produced equally good dulling surfaces but required about 3 to 4 secondsadditional immersion time. By excellent dulling is meant reflectivity isalmost totally eliminated as indicated by visual observation.

If the bath temperature is raised above C. the immersion time will beless than 80 seconds when a solution as typified by Formula No. 1 isemployed. Conversely, when the bath temperature is lowered from 95 C.,the immersion time necessary to produce good dull surfaces will beincreased. After immersion the article is rinsed and dried, and if adyeing operation is required, may now be taken to the dye bath withoutthe necessity of being dried.

It is apparent that through the practice of our invention, hard anodizedaluminum oxide coatings may successfully, economically and rapidly betreated to produce a dull, non-reflective, dye-receptive surface.Further, our solution will not attack the basis aluminum where exposed,such, for example, as at racking points. It is also to be understoodthat our invention is not limited to use with black dyes only, but thatother suitable dyes may be employed if desired.

We claim:

1. A method of minimizing the reflectivity of hard anodized aluminumoxide coatings on an aluminum-rich basis metal article which comprisestreating said article with an aqueous solution consisting essentially ofphosphoric acid of about 0.38 to 0.57 molarity, chromium tri-oxide ofabout 0.12 to 0.18 molarity about 0.01 weight percent of a surfaceactive agent.

2. A method according to claim 1 which comprises cleaning and degreasingsaid article prior to treating.

3. A method according to claim 1 which comprises rinsing and dyeing saidarticle subsequent to treating.

References Cited in the file of this patent UNITED STATES PATENTS2,438,877 Spruance Mar. 30, 1948 4 Jones Aug. 25, Newhard Nov. 26,Beiswanger et al. Nov. 26, Newhard et a1. Oct. 20, Newhard et al May 10,Newhard et al Oct. 30,

OTHER REFERENCES Henley: Light metals, Temple Press, London, October 101959, pages 536542.

Kurz: Journal of the Electrochemical Society, vol.

103, No. 4, April 1956.

1. A METHOD OF MINIMIZING THE REFLECTIVITY OF HARD ANODIZED ALUMINUMOXIDE COATINGS ON AN ALUMINUM-RICH BASIS METAL ARTICLE WHICH COMPRISESTREATING SAID ARTICLE WITH AN AQUEOUS SOLUTION CONSISTING ESSENTIALLY OFPHOSPHORIC ACID OF ABOUT 0.38 TO 0.57 MOLARITY, CHROMIUM TRI-OXIDE OFABOUT 0.12 TO 0.18 MOLARITY ABOUT 0.01 WEIGHT PERCENT OF A SURFACEACTIVE AGENT.